The present invention relates to a color picture device, and, more particularly, to a color picture device provided with an electron gun assembly of an in-line type for emitting three electron beams, and a deflection unit for deflecting the electron beams in horizontal and vertical directions.
Generally, in a color picture device of an in-line type, three electron beams corresponding to three colors, red, green and blue, are emitted from an electron gun assembly housed in a neck section of a tube envelope, and are focused by an electron lens of the electron gun assembly so that a beam spot of an optimum size is formed on a panel section such as a phosphor screen of the tube envelope. These electron beams are deflected by a deflection magnetic field produced by a deflection unit which, disposed around the neck and funnel sections of the tube envelope, consists of a saddle-type coil, for producing a horizontal deflection magnetic field, and a toroidal-type coil which is toroidally wound around a ring-shaped magnetic core and adapted to produce a vertical deflection magnetic field. The screen is scanned by the deflected electron beams.
More specifically, in the electron gun assembly of the prior art color picture device, the longitudinal or horizontal direction of the phosphor screen is in alignment with the arrangement direction of the electron beams, and the center beam is coincided substantially with the tube axis of the tube envelope. Scanning regions for the electron beams are essentially rectangular, and the scanning region for the center beam needs to be aligned exactly on the phosphor screen with each of the scanning regions of the two side beams. Normally, the loci of the three electron beams extend parallel to and at equal distances from one another in the region from the cathodes of the electron gun assembly to the main lens section. The side beams are given a slight tilt toward the tube axis at the main lens section, so that the three electron beams are converged on the phosphor screen. Misconvergence, if any, between the three electron beams may be corrected by, for example, multipolar magnets. The multipolar magnets are used in, for example, a color picture device which is disclosed in Japanese Patent Publication No. 45936/1971. In this color picture device, the multipolar magnets are attached to the neck section of the tube envelope, wherein there is contained the electron gun assembly.
The horizontal deflection magnetic field formed by the deflection unit is in the shape of a pincushion, while the vertical deflection magnetic field formed by the deflection unit is barrel-shaped. The three electron beams are converged throughout the entire rectangular screen. In this case, the convergence between the side beams and the center beam is subject to coma aberration, since the side and center beams are different in deflection sensitivity due to the nonuniform magnetic field. In a color picture device stated in Japanese Patent Disclosure No. 7053/1982, coma aberration is corrected by the use of magnetic field control elements which are attached to the tip portions of electron guns. These magnetic field control elements cause the three electron beams to act effectively on the nonuniform deflection magnetic fields.
However, these magnetic field control elements are subject to the following drawbacks. Referring to FIG. 1 in which the phosphor screen is viewed through an outer side of the transparent panel section of the tube envelope, the phosphor screen is horizontally scanned from its left end with the three electron beams. In this horizontal scanning, the intensity of the horizontal deflection magnetic field is reduced to zero for the electron beams reaching the center of the screen. Since each of the magnetic field control elements have a certain hysteresis characteristic, a remanent magnetization exists on each of the control elements even though the horizontal deflection magnetic field is reduced to zero. The remanent magnetization produces a magnetic field so that the center and side beams are slightly deflected even in the center of the screen. The magnetic field produced by the remanent magnetization serves to deflect the three electron beams to the right on the screen. The magnetic field acts on each side beam with higher intensity than on the center beam. In this case, even when the three electron beams are converged on the center of the screen without being deflected, the relative positions of the center beam and the side beams are deviated in the center of the screen during deflection scanning. Namely, in the center of the screen, the position of the center beam 4A is deviated from the position of each side beam 6A toward the horizontal scanning start side of the screen 2, as shown in FIG. 1.
In the vicinity of the short side of the screen, conversely, the three electron beams are converged by the magnetic field control elements, for forming the nonuniform magnetic field, to correct the coma aberration. This effect may be obtained because the magnetic field control elements are generally formed of material with high permeability, such as permalloy which has a low magnetic coercive force, so that the magnetic field control elements are fully saturated with magnetization produced by the deflection magnetic field. Thus, owing to remanent magnetization on each of the magnetic field control elements, that is, on account of their hysteresis characteristic, the three electron beams undergo no misconvergence in the vicinity of the short side of the screen 2, while misconvergence is caused between the center beam 4A and the two side beams 6A in the center of the screen 2. Accordingly, the multipolar magnets are adjusted so that the three electron beams converge on the center of the screen 2. This adjustment causes misconvergence in the vicinity of the short side of the screen 2, as shown in FIG. 2. Such misconvergence cannot be corrected by the magnetic field of the deflection unit. Also, in an ordinary color picture device, the size of said misconvergence ranges from 0.1 to 0.3 mm. Therefore, the picture quality, in terms of either color purity or convergence, is quite inferior. This is an important problem for a color picture device in which a high resolution is necessary.
The misconvergence between the center beam 4B' or 4C' and the side beams 6B' or 6C' in the vicinity of the short side of the screen 2 is, as shown in FIG. 2, caused by more than just the magnetic field control elements. For example, such misconvergence may also be caused by a pair of high permeability metal plates which are usually provided within the vertical deflecting coils in the neck section for the purpose of making the vertical deflection magnetic field a high-intensity barrel-shaped magnetic field. During deflection scanning, the direction, in the center of the screen, of a magnetic field produced by the high permeability metal plates is opposite to that of the remanent magnetic field produced by the magnetic field control elements. Specifically, the center and side beams are slightly deflected toward the horizontal scanning start side of the screen 2 by the magnetic field produced by the high permeability metal plates. During deflection scanning, the magnetic action on the center beam is higher in intensity than that on the side beams. In the center of the screen 2, therefore, the position of the center beam 4A is deviated from the position of each side beam 6A toward the horizontal scanning start side of the screen 2, as shown in FIG. 1. In the vicinity of the short side of the screen 2, in contrast, the three electron beams are converged by the magnetic field control elements which form the nonuniform magnetic field to correct coma aberration. In this case, the picture pattern is similar to the one caused by the magnetic field control elements, as mentioned above. This picture pattern is corrected by the multipolar magnets, so that misconvergence is caused between the center beam 4B' or 4C' and the side beams 6B' or 6C' in the vicinity of the short side of the screen 2.